1. Introduction

A virtual table is an object that is registered with an open SQLite
database connection. From the perspective of an SQL statement,
the virtual table object looks like any other table or view.
But behind the scenes, queries and updates on a virtual table
invoke callback methods of the virtual table object instead of
reading and writing on the database file.

The virtual table mechanism allows an application to publish
interfaces that are accessible from SQL statements as if they were
tables. SQL statements can do almost anything to a
virtual table that they can do to a real table, with the following
exceptions:

One cannot create a trigger on a virtual table.

One cannot create additional indices on a virtual table.
(Virtual tables can have indices but that must be built into
the virtual table implementation. Indices cannot be added
separately using CREATE INDEX statements.)

Individual virtual table implementations might impose additional
constraints. For example, some virtual implementations might provide
read-only tables. Or some virtual table implementations might allow
INSERT or DELETE but not UPDATE. Or some virtual table implementations
might limit the kinds of UPDATEs that can be made.

A virtual table might represent an in-memory data structures.
Or it might represent a view of data on disk that is not in the
SQLite format. Or the application might compute the content of the
virtual table on demand.

1.1. Usage

The CREATE VIRTUAL TABLE statement creates a new table
called table-name derived from the class
class module-name. The module-name
is the name that is registered for the virtual table by
the sqlite3_create_module() interface.

CREATE VIRTUAL TABLE tablename USING modulename;

One can also provide comma-separated arguments to the module following
the module name:

CREATE VIRTUAL TABLE tablename USING modulename(arg1, arg2, ...);

The format of the arguments to the module is very general. Each
module-argument
may contain keywords, string literals, identifiers, numbers, and
punctuation. Each module-argument is passed as
written (as text) into the
constructor method of the virtual table implementation
when the virtual
table is created and that constructor is responsible for parsing and
interpreting the arguments. The argument syntax is sufficiently general
that a virtual table implementation can, if it wants to, interpret its
arguments as column definitions in an ordinary CREATE TABLE statement.
The implementation could also impose some other interpretation on the
arguments.

Once a virtual table has been created, it can be used like any other
table with the exceptions noted above and imposed by specific virtual
table implementations. A virtual table is destroyed using the ordinary
DROP TABLE syntax.

1.1.1. Temporary virtual tables

There is no "CREATE TEMP VIRTUAL TABLE" statement. To create a
temporary virtual table, add the "temp" schema
before the virtual table name.

CREATE VIRTUAL TABLE temp.tablename USING module(arg1, ...);

1.1.2. Eponymous virtual tables

Some virtual tables exist automatically in the "main" schema of
every database connection in which their
module is registered, even without a CREATE VIRTUAL TABLE statement.
Such virtual tables are called "eponymous virtual tables".
To use an eponymous virtual table, simply use the
module name as if it were a table.
Eponymous virtual tables exist in the "main" schema only, so they will
not work if prefixed with a different schema name.

An example of an eponymous virtual table is the dbstat virtual table.
To use the dbstat virtual table as an eponymous virtual table,
simply query against the "dbstat"
module name, as if it were an ordinary table. (Note that SQLite
must be compiled with the SQLITE_ENABLE_DBSTAT_VTAB option to include
the dbstat virtual table in the build.)

SELECT * FROM dbstat;

A virtual table is eponymous if its xCreate method is the exact same
function as the xConnect method, or if the xCreate method is NULL.
The xCreate method is called when a virtual table is first created
using the CREATE VIRTUAL TABLE statement. The xConnect method whenever
a database connection attaches to or reparses a schema. When these two methods
are the same, that indicates that the virtual table has no persistent
state that needs to be created and destroyed.

Note that prior to version 3.9.0 (2015-10-14),
SQLite did not check the xCreate method
for NULL before invoking it. So if an eponymous-only virtual table is
registered with SQLite version 3.8.11.1 (2015-07-29)
or earlier and a CREATE VIRTUAL TABLE
command is attempted against that virtual table module, a jump to a NULL
pointer will occur, resulting in a crash.

1.2. Implementation

Several new C-level objects are used by the virtual table implementation:

The sqlite3_module structure defines a module object used to implement
a virtual table. Think of a module as a class from which one can
construct multiple virtual tables having similar properties. For example,
one might have a module that provides read-only access to
comma-separated-value (CSV) files on disk. That one module can then be
used to create several virtual tables where each virtual table refers
to a different CSV file.

The module structure contains methods that are invoked by SQLite to
perform various actions on the virtual table such as creating new
instances of a virtual table or destroying old ones, reading and
writing data, searching for and deleting, updating, or inserting rows.
The module structure is explained in more detail below.

Each virtual table instance is represented by an sqlite3_vtab structure.
The sqlite3_vtab structure looks like this:

Virtual table implementations will normally subclass this structure
to add additional private and implementation-specific fields.
The nRef field is used internally by the SQLite core and should not
be altered by the virtual table implementation. The virtual table
implementation may pass error message text to the core by putting
an error message string in zErrMsg.
Space to hold this error message string must be obtained from an
SQLite memory allocation function such as sqlite3_mprintf() or
sqlite3_malloc().
Prior to assigning a new value to zErrMsg, the virtual table
implementation must free any preexisting content of zErrMsg using
sqlite3_free(). Failure to do this will result in a memory leak.
The SQLite core will free and zero the content of zErrMsg when it
delivers the error message text to the client application or when
it destroys the virtual table. The virtual table implementation only
needs to worry about freeing the zErrMsg content when it overwrites
the content with a new, different error message.

The sqlite3_vtab_cursor structure represents a pointer to a specific
row of a virtual table. This is what an sqlite3_vtab_cursor looks like:

The sqlite3_create_module() and sqlite3_create_module_v2()
routines associates a module name with
an sqlite3_module structure and a separate client data that is specific
to each module. The only difference between the two create_module methods
is that the _v2 method includes an extra parameter that specifies a
destructor for client data pointer. The module structure is what defines
the behavior of a virtual table. The module structure looks like this:

The module structure defines all of the methods for each virtual
table object. The module structure also contains the iVersion field which
defines the particular edition of the module table structure. Currently,
iVersion is always 1, but in future releases of SQLite the module structure
definition might be extended with additional methods and in that case
the iVersion value will be increased.

The rest of the module structure consists of methods used to implement
various features of the virtual table. Details on what each of these
methods do are provided in the sequel.

The only really hard part is step 1. You might want to start with an
existing virtual table implementation and modify it to suit your needs.
There are several virtual table implementations in the SQLite source tree
(for testing purposes). You might use one of those as a guide. Locate
these test virtual table implementations by searching
for "sqlite3_create_module".

The db parameter is a pointer to the SQLite database connection that
is executing the CREATE VIRTUAL TABLE statement.
The pAux argument is the copy of the client data pointer that was the
fourth argument to the sqlite3_create_module() or
sqlite3_create_module_v2() call that registered the
virtual table module.
The argv parameter is an array of argc pointers to null terminated strings.
The first string, argv[0], is the name of the module being invoked. The
module name is the name provided as the second argument to
sqlite3_create_module() and as the argument to the USING clause of the
CREATE VIRTUAL TABLE statement that is running.
The second, argv[1], is the name of the database in which the new virtual table is being created. The database name is "main" for the primary database, or
"temp" for TEMP database, or the name given at the end of the ATTACH
statement for attached databases. The third element of the array, argv[2],
is the name of the new virtual table, as specified following the TABLE
keyword in the CREATE VIRTUAL TABLE statement.
If present, the fourth and subsequent strings in the argv[] array report
the arguments to the module name in the CREATE VIRTUAL TABLE statement.

The job of this method is to construct the new virtual table object
(an sqlite3_vtab object) and return a pointer to it in *ppVTab.

The first argument to sqlite3_declare_vtab() must be the same
database connection pointer as the first parameter to this method.
The second argument to sqlite3_declare_vtab() must a zero-terminated
UTF-8 string that contains a well-formed CREATE TABLE statement that
defines the columns in the virtual table and their data types.
The name of the table in this CREATE TABLE statement is ignored,
as are all constraints. Only the column names and datatypes matter.
The CREATE TABLE statement string need not to be
held in persistent memory. The string can be
deallocated and/or reused as soon as the sqlite3_declare_vtab()
routine returns.

The xCreate method need not initialize the pModule, nRef, and zErrMsg
fields of the sqlite3_vtab object. The SQLite core will take care of
that chore.

The xCreate should return SQLITE_OK if it is successful in
creating the new virtual table, or SQLITE_ERROR if it is not successful.
If not successful, the sqlite3_vtab structure must not be allocated.
An error message may optionally be returned in *pzErr if unsuccessful.
Space to hold the error message string must be allocated using
an SQLite memory allocation function like
sqlite3_malloc() or sqlite3_mprintf() as the SQLite core will
attempt to free the space using sqlite3_free() after the error has
been reported up to the application.

If the xCreate method is omitted (left as a NULL pointer) then the
virtual table is an eponymous-only virtual table. New instances of
the virtual table cannot be created using CREATE VIRTUAL TABLE and the
virtual table can only be used via its module name.
Note that SQLite versions prior to 3.9.0 (2015-10-14) do not understand
eponymous-only virtual tables and will segfault if an attempt is made
to CREATE VIRTUAL TABLE on an eponymous-only virtual table because
the xCreate method was not checked for null.

If the xCreate method is the exact same pointer as the xConnect method,
that indicates that the virtual table does not need to initialize backing
store. Such a virtual table can be used as an eponymous virtual table
or as a named virtual table using CREATE VIRTUAL TABLE or both.

2.1.1. Hidden columns in virtual tables

If a column datatype contains the special keyword "HIDDEN"
(in any combination of upper and lower case letters) then that keyword
it is omitted from the column datatype name and the column is marked
as a hidden column internally.
A hidden column differs from a normal column in three respects:

2.1.2. Table-valued functions

A virtual table that contains hidden columns can be used like
a table-valued function in the FROM clause of a SELECT statement.
The arguments to the table-valued function become constraints on
the HIDDEN columns of the virtual table.

The sqlite3_module.xBestIndex method in the implementation of this
table checks for equality constraints against the HIDDEN columns, and uses
those as input parameters to determine the range of integer "value" outputs
to generate. Reasonable defaults are used for any unconstrained columns.
For example, to list all integers between 5 and 50:

SELECT value FROM generate_series(5,50);

The previous query is equivalent to the following:

SELECT value FROM generate_series WHERE start=5 AND stop=50;

Arguments on the virtual table name are matched to hidden columns
in order. The number of arguments can be less than the
number of hidden columns, in which case the latter hidden columns are
unconstrained. However, an error results if there are more arguments
than there are hidden columns in the virtual table.

2.1.3. WITHOUT ROWID Virtual Tables

Beginning with SQLite version 3.14.0 (2016-08-08),
the CREATE TABLE statement that
is passed into sqlite3_declare_vtab() may contain a WITHOUT ROWID clause.
This is useful for cases where the virtual table rows
cannot easily be mapped into unique integers. A CREATE TABLE
statement that includes WITHOUT ROWID must define one or more columns as
the PRIMARY KEY. Every column of the PRIMARY KEY must individually be
NOT NULL and all columns for each row must be collectively unique.

Note that SQLite does not enforce the PRIMARY KEY for a WITHOUT ROWID
virtual table. Enforcement is the responsibility of the underlying
virtual table implementation. But SQLite does assume that the PRIMARY KEY
constraint is valid - that the identified columns really are UNIQUE and
NOT NULL - and it uses that assumption to optimize queries against the
virtual table.

The rowid column is not accessible on a
WITHOUT ROWID virtual table (of course).

The xUpdate method was originally designed around having a
ROWID as a single value. The xUpdate method has been expanded to
accommodate an arbitrary PRIMARY KEY in place of the ROWID, but the
PRIMARY KEY must still be only one column. For this reason, SQLite
will reject any WITHOUT ROWID virtual table that has more than one
PRIMARY KEY column and a non-NULL xUpdate method.

The difference is that xConnect is called to establish a new
connection to an existing virtual table whereas xCreate is called
to create a new virtual table from scratch.

The xCreate and xConnect methods are only different when the
virtual table has some kind of backing store that must be initialized
the first time the virtual table is created. The xCreate method creates
and initializes the backing store. The xConnect method just connects
to an existing backing store. When xCreate and xConnect are the same,
the table is an eponymous virtual table.

As an example, consider a virtual table implementation that
provides read-only access to existing comma-separated-value (CSV)
files on disk. There is no backing store that needs to be created
or initialized for such a virtual table (since the CSV files already
exist on disk) so the xCreate and xConnect methods will be identical
for that module.

Another example is a virtual table that implements a full-text index.
The xCreate method must create and initialize data structures to hold
the dictionary and posting lists for that index. The xConnect method,
on the other hand, only has to locate and use an existing dictionary
and posting lists that were created by a prior xCreate call.

The xConnect method must return SQLITE_OK if it is successful
in creating the new virtual table, or SQLITE_ERROR if it is not
successful. If not successful, the sqlite3_vtab structure must not be
allocated. An error message may optionally be returned in *pzErr if
unsuccessful.
Space to hold the error message string must be allocated using
an SQLite memory allocation function like
sqlite3_malloc() or sqlite3_mprintf() as the SQLite core will
attempt to free the space using sqlite3_free() after the error has
been reported up to the application.

The xConnect method is required for every virtual table implementation,
though the xCreate and xConnect pointers of the sqlite3_module object
may point to the same function if the virtual table does not need to
initialize backing store.

2.3. The xBestIndex Method

SQLite uses the xBestIndex method of a virtual table module to determine
the best way to access the virtual table.
The xBestIndex method has a prototype like this:

int (*xBestIndex)(sqlite3_vtab *pVTab, sqlite3_index_info*);

The SQLite core communicates with the xBestIndex method by filling
in certain fields of the sqlite3_index_info structure and passing a
pointer to that structure into xBestIndex as the second parameter.
The xBestIndex method fills out other fields of this structure which
forms the reply. The sqlite3_index_info structure looks like this:

Note the warnings on the "estimatedRows", "idxFlags", and colUsed fields.
These fields were added with SQLite versions 3.8.2, 3.9.0, and 3.10.0, respectively.
Any extension that reads or writes these fields must first check that the
version of the SQLite library in use is greater than or equal to appropriate
version - perhaps comparing the value returned from sqlite3_libversion_number()
against constants 3008002, 3009000, and/or 3010000. The result of attempting
to access these fields in an sqlite3_index_info structure created by an
older version of SQLite are undefined.

The SQLite core calls the xBestIndex method when it is compiling a query
that involves a virtual table. In other words, SQLite calls this method
when it is running sqlite3_prepare() or the equivalent.
By calling this method, the
SQLite core is saying to the virtual table that it needs to access
some subset of the rows in the virtual table and it wants to know the
most efficient way to do that access. The xBestIndex method replies
with information that the SQLite core can then use to conduct an
efficient search of the virtual table.

While compiling a single SQL query, the SQLite core might call
xBestIndex multiple times with different settings in sqlite3_index_info.
The SQLite core will then select the combination that appears to
give the best performance.

Before calling this method, the SQLite core initializes an instance
of the sqlite3_index_info structure with information about the
query that it is currently trying to process. This information
derives mainly from the WHERE clause and ORDER BY or GROUP BY clauses
of the query, but also from any ON or USING clauses if the query is a
join. The information that the SQLite core provides to the xBestIndex
method is held in the part of the structure that is marked as "Inputs".
The "Outputs" section is initialized to zero.

The information in the sqlite3_index_info structure is ephemeral
and may be overwritten or deallocated as soon as the xBestIndex method
returns. If the xBestIndex method needs to remember any part of the
sqlite3_index_info structure, it should make a copy. Care must be
take to store the copy in a place where it will be deallocated, such
as in the idxStr field with needToFreeIdxStr set to 1.

Note that xBestIndex will always be called before xFilter, since
the idxNum and idxStr outputs from xBestIndex are required inputs to
xFilter. However, there is no guarantee that xFilter will be called
following a successful xBestIndex.

The xBestIndex method is required for every virtual table implementation.

2.3.1. Inputs

The main thing that the SQLite core is trying to communicate to
the virtual table is the constraints that are available to limit
the number of rows that need to be searched. The aConstraint[] array
contains one entry for each constraint. There will be exactly
nConstraint entries in that array.

Each constraint will correspond to a term in the WHERE clause
or in a USING or ON clause that is of the form

column OP EXPR

Where "column" is a column in the virtual table, OP is an operator
like "=" or "<", and EXPR is an arbitrary expression. So, for example,
if the WHERE clause contained a term like this:

a = 5

Then one of the constraints would be on the "a" column with
operator "=" and an expression of "5". Constraints need not have a
literal representation of the WHERE clause. The query optimizer might
make transformations to the
WHERE clause in order to extract as many constraints
as it can. So, for example, if the WHERE clause contained something
like this:

x BETWEEN 10 AND 100 AND 999>y

The query optimizer might translate this into three separate constraints:

x >= 10
x <= 100
y < 999

For each constraint, the aConstraint[].iColumn field indicates which
column appears on the left-hand side of the constraint.
The first column of the virtual table is column 0.
The rowid of the virtual table is column -1.
The aConstraint[].op field indicates which operator is used.
The SQLITE_INDEX_CONSTRAINT_* constants map integer constants
into operator values.
Columns occur in the order they were defined by the call to
sqlite3_declare_vtab() in the xCreate or xConnect method.
Hidden columns are counted when determining the column index.

The aConstraint[] array contains information about all constraints
that apply to the virtual table. But some of the constraints might
not be usable because of the way tables are ordered in a join.
The xBestIndex method must therefore only consider constraints
that have an aConstraint[].usable flag which is true.

In addition to WHERE clause constraints, the SQLite core also
tells the xBestIndex method about the ORDER BY clause.
(In an aggregate query, the SQLite core might put in GROUP BY clause
information in place of the ORDER BY clause information, but this fact
should not make any difference to the xBestIndex method.)
If all terms of the ORDER BY clause are columns in the virtual table,
then nOrderBy will be the number of terms in the ORDER BY clause
and the aOrderBy[] array will identify the column for each term
in the order by clause and whether or not that column is ASC or DESC.

In SQLite version 3.10.0 (2016-01-06) and later,
the colUsed field is available
to indicate which fields of the virtual table are actually used by the
statement being prepared. If the lowest bit of colUsed is set, that
means that the first column is used. The second lowest bit corresponds
to the second column. And so forth. If the most significant bit of
colUsed is set, that means that one or more columns other than the
first 63 columns are used. If column usage information is needed by the
xFilter method, then the required bits must be encoded into either
the idxNum or idxStr output fields.

2.3.2. Outputs

Given all of the information above, the job of the xBestIndex
method it to figure out the best way to search the virtual table.

The xBestIndex method fills the idxNum and idxStr fields with
information that communicates an indexing strategy to the xFilter
method. The information in idxNum and idxStr is arbitrary as far
as the SQLite core is concerned. The SQLite core just copies the
information through to the xFilter method. Any desired meaning can
be assigned to idxNum and idxStr as long as xBestIndex and xFilter
agree on what that meaning is.

The idxStr value may be a string obtained from an SQLite
memory allocation function such as sqlite3_mprintf().
If this is the case, then the needToFreeIdxStr flag must be set to
true so that the SQLite core will know to call sqlite3_free() on
that string when it has finished with it, and thus avoid a memory leak.

If the virtual table will output rows in the order specified by
the ORDER BY clause, then the orderByConsumed flag may be set to
true. If the output is not automatically in the correct order
then orderByConsumed must be left in its default false setting.
This will indicate to the SQLite core that it will need to do a
separate sorting pass over the data after it comes out of the virtual table.

The estimatedCost field should be set to the estimated number
of disk access operations required to execute this query against
the virtual table. The SQLite core will often call xBestIndex
multiple times with different constraints, obtain multiple cost
estimates, then choose the query plan that gives the lowest estimate.

If the current version of SQLite is 3.8.2 or greater, the estimatedRows
field may be set to an estimate of the number of rows returned by the
proposed query plan. If this value is not explicitly set, the default
estimate of 25 rows is used.

If the current version of SQLite is 3.9.0 or greater, the idxFlags field
may be set to SQLITE_INDEX_SCAN_UNIQUE to indicate that the virtual table
will return only zero or one rows given the input constraints. Additional
bits of the idxFlags field might be understood in later versions of SQLite.

The aConstraintUsage[] array contains one element for each of
the nConstraint constraints in the inputs section of the
sqlite3_index_info structure.
The aConstraintUsage[] array is used by xBestIndex to tell the
core how it is using the constraints.

The xBestIndex method may set aConstraintUsage[].argvIndex
entries to values greater than zero.
Exactly one entry should be set to 1, another to 2, another to 3,
and so forth up to as many or as few as the xBestIndex method wants.
The EXPR of the corresponding constraints will then be passed
in as the argv[] parameters to xFilter.

For example, if the aConstraint[3].argvIndex is set to 1, then
when xFilter is called, the argv[0] passed to xFilter will have
the EXPR value of the aConstraint[3] constraint.

By default, the SQLite core double checks all constraints on
each row of the virtual table that it receives. If such a check
is redundant, the xBestFilter method can suppress that double-check by
setting aConstraintUsage[].omit.

2.4. The xDisconnect Method

int (*xDisconnect)(sqlite3_vtab *pVTab);

This method releases a connection to a virtual table.
Only the sqlite3_vtab object is destroyed.
The virtual table is not destroyed and any backing store
associated with the virtual table persists.
This method undoes the work of xConnect.

This method is a destructor for a connection to the virtual table.
Contrast this method with xDestroy. The xDestroy is a destructor
for the entire virtual table.

The xDisconnect method is required for every virtual table implementation,
though it is acceptable for the xDisconnect and xDestroy methods to be
the same function if that makes sense for the particular virtual table.

2.5. The xDestroy Method

int (*xDestroy)(sqlite3_vtab *pVTab);

This method releases a connection to a virtual table, just like
the xDisconnect method, and it also destroys the underlying
table implementation. This method undoes the work of xCreate.

The xDisconnect method is called whenever a database connection
that uses a virtual table is closed. The xDestroy method is only
called when a DROP TABLE statement is executed against the virtual table.

The xDestroy method is required for every virtual table implementation,
though it is acceptable for the xDisconnect and xDestroy methods to be
the same function if that makes sense for the particular virtual table.

2.6. The xOpen Method

int (*xOpen)(sqlite3_vtab *pVTab, sqlite3_vtab_cursor **ppCursor);

The xOpen method creates a new cursor used for accessing (read and/or
writing) a virtual table. A successful invocation of this method
will allocate the memory for the sqlite3_vtab_cursor (or a subclass),
initialize the new object, and make *ppCursor point to the new object.
The successful call then returns SQLITE_OK.

For every successful call to this method, the SQLite core will
later invoke the xClose method to destroy
the allocated cursor.

The xOpen method need not initialize the pVtab field of the
sqlite3_vtab_cursor structure. The SQLite core will take care
of that chore automatically.

A virtual table implementation must be able to support an arbitrary
number of simultaneously open cursors.

When initially opened, the cursor is in an undefined state.
The SQLite core will invoke the xFilter method
on the cursor prior to any attempt to position or read from the cursor.

The xOpen method is required for every virtual table implementation.

2.7. The xClose Method

int (*xClose)(sqlite3_vtab_cursor*);

The xClose method closes a cursor previously opened by
xOpen.
The SQLite core will always call xClose once for each cursor opened
using xOpen.

This method must release all resources allocated by the
corresponding xOpen call. The routine will not be called again even if it
returns an error. The SQLite core will not use the
sqlite3_vtab_cursor again after it has been closed.

The xClose method is required for every virtual table implementation.

2.8. The xEof Method

int (*xEof)(sqlite3_vtab_cursor*);

The xEof method must return false (zero) if the specified cursor
currently points to a valid row of data, or true (non-zero) otherwise.
This method is called by the SQL engine immediately after each
xFilter and xNext invocation.

The xEof method is required for every virtual table implementation.

2.9. The xFilter Method

This method begins a search of a virtual table.
The first argument is a cursor opened by xOpen.
The next two arguments define a particular search index previously
chosen by xBestIndex. The specific meanings of idxNum and idxStr
are unimportant as long as xFilter and xBestIndex agree on what
that meaning is.

The xBestIndex function may have requested the values of
certain expressions using the aConstraintUsage[].argvIndex values
of the sqlite3_index_info structure.
Those values are passed to xFilter using the argc and argv parameters.

If the virtual table contains one or more rows that match the
search criteria, then the cursor must be left point at the first row.
Subsequent calls to xEof must return false (zero).
If there are no rows match, then the cursor must be left in a state
that will cause the xEof to return true (non-zero).
The SQLite engine will use
the xColumn and xRowid methods to access that row content.
The xNext method will be used to advance to the next row.

The xFilter method is required for every virtual table implementation.

2.10. The xNext Method

int (*xNext)(sqlite3_vtab_cursor*);

The xNext method advances a virtual table cursor
to the next row of a result set initiated by xFilter.
If the cursor is already pointing at the last row when this
routine is called, then the cursor no longer points to valid
data and a subsequent call to the xEof method must return true (non-zero).
If the cursor is successfully advanced to another row of content, then
subsequent calls to xEof must return false (zero).

2.11. The xColumn Method

int (*xColumn)(sqlite3_vtab_cursor*, sqlite3_context*, int N);

The SQLite core invokes this method in order to find the value for
the N-th column of the current row. N is zero-based so the first column
is numbered 0.
The xColumn method may return its result back to SQLite using one of the
following interface:

If the xColumn method implementation calls none of the functions above,
then the value of the column defaults to an SQL NULL.

To raise an error, the xColumn method should use one of the result_text()
methods to set the error message text, then return an appropriate
error code. The xColumn method must return SQLITE_OK on success.

The xColumn method is required for every virtual table implementation.

2.12. The xRowid Method

int (*xRowid)(sqlite3_vtab_cursor *pCur, sqlite_int64 *pRowid);

A successful invocation of this method will cause *pRowid to be
filled with the rowid of row that the
virtual table cursor pCur is currently pointing at.
This method returns SQLITE_OK on success.
It returns an appropriate error code on failure.

2.13. The xUpdate Method

All changes to a virtual table are made using the xUpdate method.
This one method can be used to insert, delete, or update.

The argc parameter specifies the number of entries in the argv array.
The value of argc will be 1 for a pure delete operation or N+2 for an insert
or replace or update where N is the number of columns in the table.
In the previous sentence, N includes any hidden columns.

Every argv entry will have a non-NULL value in C but may contain the
SQL value NULL. In other words, it is always true that
argv[i]!=0 for i between 0 and argc-1.
However, it might be the case that
sqlite3_value_type(argv[i])==SQLITE_NULL.

The argv[0] parameter is the rowid of a row in the virtual table
to be deleted. If argv[0] is an SQL NULL, then no deletion occurs.

The argv[1] parameter is the rowid of a new row to be inserted
into the virtual table. If argv[1] is an SQL NULL, then the implementation
must choose a rowid for the newly inserted row. Subsequent argv[]
entries contain values of the columns of the virtual table, in the
order that the columns were declared. The number of columns will
match the table declaration that the xConnect or xCreate method made
using the sqlite3_declare_vtab() call. All hidden columns are included.

When doing an insert without a rowid (argc>1, argv[1] is an SQL NULL),
on a virtual table that uses ROWID (but not on a WITHOUT ROWID virtual table,
the implementation must set *pRowid to the rowid of the newly inserted row;
this will become the value returned by the sqlite3_last_insert_rowid()
function. Setting this value in all the other cases is a harmless no-op;
the SQLite engine ignores the *pRowid return value if argc==1 or
argv[1] is not an SQL NULL.

Each call to xUpdate will fall into one of cases shown below.
Not that references to argv[i] mean the SQL value
held within the argv[i] object, not the argv[i]
object itself.

argc = 1 argv[0] ≠ NULL

The single row with rowid or PRIMARY KEY equal to argv[0] is deleted.
No insert occurs.

argc > 1 argv[0] = NULL

A new row is inserted with column values taken from
argv[2] and following. In a rowid virtual table, if argv[1] is an SQL NULL,
then a new unique rowid is generated automatically. The argv[1] will be NULL
for a WITHOUT ROWID virtual table, in which case the implementation should
take the PRIMARY KEY value from the appropiate column in argv[2] and following.

argc > 1 argv[0] ≠ NULL argv[0] = argv[1]

The row with rowid or PRIMARY KEY argv[0] is updated with new values
in argv[2] and following parameters.

argc > 1 argv[0] ≠ NULL argv[0] ≠ argv[1]

The row with rowid or PRIMARY KEY argv[0] is updated with
the rowid or PRIMARY KEY in argv[1]
and new values in argv[2] and following parameters. This will occur
when an SQL statement updates a rowid, as in the statement:

The xUpdate method must return SQLITE_OK if and only if it is
successful. If a failure occurs, the xUpdate must return an appropriate
error code. On a failure, the pVTab->zErrMsg element may optionally
be replaced with error message text stored in memory allocated from SQLite
using functions such as sqlite3_mprintf() or sqlite3_malloc().

If the xUpdate method violates some constraint of the virtual table
(including, but not limited to, attempting to store a value of the wrong
datatype, attempting to store a value that is too
large or too small, or attempting to change a read-only value) then the
xUpdate must fail with an appropriate error code.

There might be one or more sqlite3_vtab_cursor objects open and in use
on the virtual table instance and perhaps even on the row of the virtual
table when the xUpdate method is invoked. The implementation of
xUpdate must be prepared for attempts to delete or modify rows of the table
out from other existing cursors. If the virtual table cannot accommodate
such changes, the xUpdate method must return an error code.

The xUpdate method is optional.
If the xUpdate pointer in the sqlite3_module for a virtual table
is a NULL pointer, then the virtual table is read-only.

2.14. The xFindFunction Method

This method is called during sqlite3_prepare() to give the virtual
table implementation an opportunity to overload functions.
This method may be set to NULL in which case no overloading occurs.

When a function uses a column from a virtual table as its first
argument, this method is called to see if the virtual table would
like to overload the function. The first three parameters are inputs:
the virtual table, the number of arguments to the function, and the
name of the function. If no overloading is desired, this method
returns 0. To overload the function, this method writes the new
function implementation into *pxFunc and writes user data into *ppArg
and returns 1.

Note that infix functions (LIKE, GLOB, REGEXP, and MATCH) reverse
the order of their arguments. So "like(A,B)" is equivalent to "B like A".
For the form "B like A" the B term is considered the first argument
to the function. But for "like(A,B)" the A term is considered the
first argument.

The function pointer returned by this routine must be valid for
the lifetime of the sqlite3_vtab object given in the first parameter.

2.15. The xBegin Method

int (*xBegin)(sqlite3_vtab *pVTab);

This method begins a transaction on a virtual table.
This is method is optional. The xBegin pointer of sqlite3_module
may be NULL.

This method is always followed by one call to either the
xCommit or xRollback method. Virtual table transactions do
not nest, so the xBegin method will not be invoked more than once
on a single virtual table
without an intervening call to either xCommit or xRollback.
Multiple calls to other methods can and likely will occur in between
the xBegin and the corresponding xCommit or xRollback.

2.16. The xSync Method

int (*xSync)(sqlite3_vtab *pVTab);

This method signals the start of a two-phase commit on a virtual
table.
This is method is optional. The xSync pointer of sqlite3_module
may be NULL.

This method is only invoked after call to the xBegin method and
prior to an xCommit or xRollback. In order to implement two-phase
commit, the xSync method on all virtual tables is invoked prior to
invoking the xCommit method on any virtual table. If any of the
xSync methods fail, the entire transaction is rolled back.

2.17. The xCommit Method

int (*xCommit)(sqlite3_vtab *pVTab);

This method causes a virtual table transaction to commit.
This is method is optional. The xCommit pointer of sqlite3_module
may be NULL.

A call to this method always follows a prior call to xBegin and
xSync.

2.18. The xRollback Method

int (*xRollback)(sqlite3_vtab *pVTab);

This method causes a virtual table transaction to rollback.
This is method is optional. The xRollback pointer of sqlite3_module
may be NULL.

2.19. The xRename Method

int (*xRename)(sqlite3_vtab *pVtab, const char *zNew);

This method provides notification that the virtual table implementation
that the virtual table will be given a new name.
If this method returns SQLITE_OK then SQLite renames the table.
If this method returns an error code then the renaming is prevented.

The xRename method is required for every virtual table implementation.

2.20. The xSavepoint, xRelease, and xRollbackTo Methods

These methods provide the virtual table implementation an opportunity to
implement nested transactions. They are always optional and will only be
called in SQLite version 3.7.7 (2011-06-23) and later.

R-01448-06859:[When xSavepoint(X,N) is invoked, that is a signal to the virtual table X
that it should save its current state as savepoint N.]R-42049-24001:[A subsequent call
to xRollbackTo(X,R) means that the state of the virtual table should return
to what it was when xSavepoint(X,R) was last called.]R-42243-62806:[The call
to xRollbackTo(X,R) will invalidate all savepoints with N>R; none of the
invalided savepoints will be rolled back or released without first
being reinitialized by a call to xSavepoint().]R-38513-43475:[A call to xRelease(X,M) invalidates all savepoints where N>=M.]

R-49759-12456:[None of the xSavepoint(), xRelease(), or xRollbackTo() methods will ever
be called except in between calls to xBegin() and
either xCommit() or xRollback().]